Remove converting to negative fd

Convert a fd to -(fd + 1) if its a Fixnum seems wrong.
Negative fds's are mostly invalid.

Author: tak1n

kernel/common/process_mirror.rb

@@ -186,7 +186,6 @@ def convert_to_fd(obj, target)
             when 2
               if obj[0] == :child
                 fd = convert_to_fd obj[1], target
-                fd.kind_of?(::Fixnum) ?  -(fd + 1) : fd
               else
                 [obj[0], convert_file_mode(obj[1]), 0644]
               end